EXCHANGE 


THE  UNIVERSITY  OF  CHICAGO 

MULTIPLE  VALENCY  IN 
THE  IONIZATION  BY  ALPHA  RAYS 


A  DISSERTATION 

SUBMITTED  TO  THE  FACULTY 

OF  THE  OGDEN  GRADUATE  SCHOOL  OF  SCIENCE 

IN  CANDIDACY  FOR  THE  DEGREE  OF 

DOCTOR  OF  PHILOSOPHY 


DEPARTMENT  OF  PHYSICS 


BY 

THOMAS  RUSSELL  WILKINS 


CHICAGO 

SEPTEMBER,    IQ2I 


THE  UNIVERSITY  OF  CHICAGO 

MULTIPLE  VALENCY  IN 
THE  IONIZAT1ON  BY  ALPHA  RAYS 


A  DISSERTATION 

SUBMITTED  TO  THE  FACULTY 

OF  THE  OGDEN  GRADUATE  SCHOOL  OF  SCIENCE 

IN  CANDIDACY  FOR  THE  DEGREE  OF 

DOCTOR  OF  PHILOSOPHY 


DEPARTMENT  OF  PHYSICS 


BY 

THOMAS  RUSSELL  WILKINS 


CHICAGO 

SEPTEMBER,    1 92 1 


210  T.   R.    WILKINS. 


MULTIPLE  VALENCY  IN  THE  IONIZATION  BY  ALPHA  RAYS. 

BY  T.  R.  WILKINS. 

SYNOPSIS. 

Valency  of  Ions  Produced  by  a-rays. — The  well-known  Millikan  oil-drop  method, 
in  which  the  valency  of  individual  ions  caught  on  the  droplet  immediately  after  they 
are  produced  is  determined  by  the  change  of  speed  of  the  droplet  due  to  each,  was 
used  with  slight  modifications  which  simplified  the  procedure.  In  air,  out  of  350 
positive  ions  produced  by  x-rays  at  the  end  of  their  range,  only  3  per  cent,  could 
have  been  doubly  charged  and  most  if  not  all  of  these  were  probably  not  doubles 
but  two  successive  singles.  In  helium,  an  extensive  series  of  2,150  positive  catches 
were  observed  with  gradually  increasing  pressure  to  determine  not  only  whether 
doubles  were  produced  but  also  the  variation  of  the  number  of  doubles  with  the  range. 
It  was  found  that  an  appreciable  proportion  of  real  doubles  is  produced  when  the 
ranges  of  the  rays  are  equivalent  to  between  4  and  30  mm.  of  helium  at  atmospheric 
pressure,  the  maximum  proportion  being  about  10  per  cent.,  for  the  range  of  maxi- 
mum ionizing  power.  That  this  result  was  real  was  proved  by  the  fact  that  when 
hydrogen  was  substituted,  even  for  the  range  of  maximum  ionization,  the  proportion 
of  apparent  doubles  was  only  5  out  of  200  positive  ions.  The  previously  reported 
negative  result  for  mercury  dimethyl  was  confirmed. 

Relative  stopping  power  for  ct-rays,  of  air  and  helium,  came  out  close  to  3.8,  in 
agreement  with  the  Bragg- Kleeman  law. 

INTRODUCTION. 

THE  nature  of  the  process  of  the  ionization  of  gases  by  alpha  rays 
was  investigated  by  Millikan,  Gottschalk,  and  Kelly 1  by  catching 
an  ionized  molecule  upon  an  oil-drop  at  the  instant  of  ionization  and  then 
measuring  the  charge  thus  added  to  the  drop.  The  results  prove  that 
at  least  99  times  out  of  100,  ionization  by  an  alpha  ray,  in  the  case  of  the 
following  gases  and  vapors  (air,  carbon  dioxide,  carbon  tetrachloride, 
methyl  iodide,  and  mercury  dimethyl),  consists  in  the  detachment  of  a 
single  electron  from  a  molecule. 

These  results,  along  with  much  other  work,  summarized  by  Millikan, 
Gottschalk  and  Kelly,  point  to  the  formation  of  univalent  ions.  Indeed 
the  only  evidence  for  the  formation  of  multivalent  ions  was  the  positive 
ray  work  of  Sir  J.  J.  Thomson.2  Millikan  pointed  out,  however,  that 
these  results  were  not  at  all  irreconcilable,  as  a  consideration  of  the 
velocities  and  the  forces  involved,  made  it  conceivable  that  slow  positive 
rays  might  make  multivalent  ions  when  fast  alpha  rays  would  not.  It 

»  R.  A.  Millikan,  V.  H.  Gottschalk,  M.  J.  Kelly,  PHYS.  REV.,  15,  157,  1920. 
8  J.  J.  Thomson,  Rays  of  Positive  Electricity  and  their  Application  to  Chemical  Analysis 
Longmans,  Green  &  Co. 


X'l     MULTIPLE    VALENCY  IN   IONIZATION  BY   ALPHA    RAYS.       211 

became  desirable,  therefore,  to  test  this  point  and,  accordingly,  such 
work  has  been  undertaken. 

Reference  was  also  made  to  the  scintillation  experiments  of  Sir  Ernest 
Rutherford  on  the  "swift  atoms"  resulting  from  ionizing  gases  by  the 
nuclear  impact  of  alpha  rays,  in  which  the  inference  was  drawn  from 
negative,  rather  than  positive  evidence,  that  doubly  charged  ions  of 
helium  were  formed.  The  importance  of  extending  the  oil-drop  experi- 
ments to  include  a  study  of  the  alpha-ray  ionization  of  helium  was  obvious, 
both  because  of  the  fundamental  role  played  by  the  helium  atom  in  the 
present-day  theories  of  atomic  structure  and  the  fact  that  in  alpha  rays 
themselves  we  really  have  doubly  charged  helium  ions.  Recently  it  has 
become  possible  to  secure  helium  in  sufficient  quantities  for  use  in  the 
oil-drop  apparatus,  the  chamber  of  which  has  a  volume  of  about  twenty 
litres,  and  the  results  of  a  study  of  the  alpha-ray  ionization  of  helium  are 
here  given. 

The  results  in  helium,  as  will  appear,  made  it  seem  desirable  to  extend 
the  work  to  hydrogen,  and  this  has  also  been  done. 

APPARATUS. 

The  apparatus  used  was  identical  with  that  already  described  l  except 
that  polonium  was  used  instead  of  radium  bromide  as  a  source  of  alpha 
rays.  Unfortunately  polonium  was  not  available  at  the  time  of  the  earlier 
work.  While  not  altering  the  procedure  in  any  way,  this  substitution 
brought  two  advantages:  first,  it  removed  the  annoyances  encountered 
in  the  earlier  work  which  were  caused  by  the  formation  of  an  "active 
deposit"  on  the  walls  of  the  chamber,  and  the  consequent  increased  and 
uncontrollable  ionization  due  to  this  cause;  secondly,  it  provided  a 
"simple"  source  of  ionizing  rays  for  which  a  random  distribution  of 
ejection  could  be  assumed  in  a  study  of  the  frequency  of  catches. 

The  polonium  was  prepared  by  Mr.  E.  T.  Johnson  and  Mr.  B.  A. 
Rogers,  who  also  conducted  some  observations  on  the  ionization  of  air, 
to  which  reference  will  be  made  later.  To  obtain  the  polonium  some 
old  radium  preparation  was  used  and  the  usual  method  followed.2  The 
product  was  tested  in  an  electroscope  for  the  presence  of  beta  rays  by 
covering  it  with  sufficient  aluminum  foil  to  absorb  the  alpha  rays  but 
not  the  beta  rays,  and  it  was  found  free  of  beta  radiation. 

The  coarse  adjustment  to  obtain  iorization  at  the  end  of  the  range 
was  made  by  interposing  absorbing  foil  of  the  necessary  thickness  and 
the  finer  adjustment  made  by  varying  the  gas  pressure  in  the  chamber. 
In  this  way  a  very  delicate  adjustment  could  be  obtained. 

1  R.  A.  Millikan,  V.  H.  Gottschalk,  M.  J.  Kelly,  loc.  cit. 

2  Practical    Measurements   in   Radioactivity,  W.    Makower  and   H.    Geiger,  Lonmgans, 
Green  &  Co.,  p.  123. 


212  T.    R.    WILKINS.  [!ER?E? 

• 

A  concentrated  filament  lamp  replaced  the  arc  used  in  the  earlier 
work  and  gave  a  very  steady  illumination  and  did  away  with  the  neces- 
sity of  frequent  adjustments  of  the  arc. 

A  potentiometer  switch  was  arranged  so  that  the  plate  voltage  could 
be  varied  through  wide  ranges  in  steps  of  15  volts. 

PROCEDURE. 

The  general  procedure  followed  was  that  already  described.1  For 
the  greatest  possible  simplicity  of  observation  it  was  always  arranged  to 
have  but  one  charge  on  the  drop  when  the  polonium  door  was  open. 

The  field  between  the  condenser  plates  was  adjusted  to  balance  the 
drop  when  it  had  this  one  charge  on  it  and  measurements  were  made 
at  this  one  voltage.  Thus  the  duties  of  the  experimenter  were  very  much 
lightened.  As  soon  as  a  catch  was  made  and  measured,  the  drop  was 
made  ready  for  another  catching  by  getting  rid  of  all  but  one  charge. 
This  was  done  in  the  usual  way. 

The  experimental  testing  of  the  number  of  charges  on  the  drop  is 
very  simple  under  this  procedure  for  there  is  then  clearly  an  exact 
multiple  relation  between  the  characteristic  speeds  corresponding  to 
various  numbers  of  charges.  And,  further,  no  time  is  wasted  in  first 
determining  the  characteristic  speeds  or  in  redetermining  them  as  the 
voltage  of  the  cells  drops,  for  if  the  voltage  drop,  the  potentiometer 
switch  is  moved  a  peg  till  the  original  balance  is  secured.  This  was  of 
help  in  the  work  on  mercury  dimethyl  vapor,  where  the  temperature 
was  raised  considerably  to  raise  the  vapor  pressure  while  a  run  was 
in  progress.  No  confusion  resulted,  for  although  the  numerical  values 
of  the  speeds  changed,  each  one  altered  so  that  a  multiple  relation  again 
held.  For  example,  the  speed  corresponding  to  a  catch  of  one  charge 
changed  from  16  to  18  seconds  as  the  temperature  and  vapor  pressure 
rose.  That  corresponding  to  a  catch  of  a  double  rose,  proportionately, 
from  8  to  9  seconds,  and  similarly,  the  triple  speed  changed  from  5.3 
to  6  seconds. 

To  express  these  relations  in  mathematical  form,  we  have  the  two 
following  equations: 

(1)  Vg  =  kmg  where  V0  =  the  vel.  of  fall  under  gravity. 

k     =  a  constant  of  proportionality. 

(2)  V    =  k(neE  —  mg)  V    =  vel.  of  rise  when  the  drop  has  a 

positive  charge. 

n     —  number  of  charges  on  drop. 
e     =  the  elementary  charge. 
E    =  the  P. D.  between  condenser  plates. 

1  Millikan,  Kelly  and  Gottschalk,  lex:,  cit. 


VOL.  XIX. 
No.  3. 


]     MULTIPLE    VALENCY  IN  IONIZATION  BY  ALPHA    RAYS.       213 


Now,  if  Ee  =  mg,  i.e.,  if  the  potential  applied  is  just  sufficient  to 
balance  a  drop  with  unit  charge,  (2)  becomes, 

(3)-  V=k(n-i)mg. 

and  so  if 

w  =  I,         7  =  0. 

n  =  2,         7  =  kmg. 

n  =  3,         7  =  2&rag. 
etc. 

thus,  when  a  charge  is  caught,  the  drop  will  start  upward  with  a  speed 
equal  to  kmg — if  two  charges  are  caught,  with  twice  that  speed;  and  if 
three  charges  are  caught,  with  three  times  that  speed,  and  we  immediately 
have  the  number  of  charges  caught  because  of  this  multiple  relationship 
in  speeds. 

As  evidence  of  the  careful  balancing  of  the  drop  and  the  control  of  tjie 
ionization,  it  may  be  mentioned  that  the  drop  was  often  left  suspended 
for  intervals  of  a  quarter  to  half  an  hour  and  in  one  case  a  drop  was  left 
for  an  hour  and  a  quarter  and  was  in  practically  the  same  spot  when  the 
observers  returned  from  dinner. 

An  attempt  was  made  to  take  as  long  runs  as  possible  on  the  same  drop 
and  in  no  case  have  fewer  than  21  catches  been  reported  on  the  work  in 
air  and  in  the  work  in  helium  not  fewer  than  50  catches  of  positives.  As 
many  as  150  catches  are  reported  on  some  drops  corresponding  to  from 
six  to  eight  hours'  observation  on  the  same  drop.  Most  of  the  drops 
remained  bright  right  up  to  the  end  of  the  run,  but  in  the  work  on  mercury 
dimethyl  vapor,  where  an  attempt  was  made  to  raise  the  vapor  pressure 
by  raising  the  temperature  some  15  or  20  deg.  C.,  the  drops  soon  lost 
their  lustre  and  only  short  runs  were  possible. 

DATA — AIR. 
TABLE  I. 


Drop 
Number. 

Pressure 
(Cm.  of  Hg). 

(Sees.). 

Total  No.  of  Catches 
(Pos.  Negatives). 

Double 
Positives. 

1 

8 

103 

95 

2 

2 

8.1 

95 

21 

0 

3 

8.1 

150 

68 

1 

4 

8.05 

102 

35 

0 

5 

8.1 

105 

108 

0 

6 

7 

8.15 
8.3 

114 
103 

66 
21 

3 
0 

8 

8.15 

138 

48 

2 

9 

8.2 

110 

52 

2 

514 

10 

214  T-    R-    WILKINS. 

Table  I.  shows  that  out  of  514  catches,  350  of  which  were  positives, 
10  were  apparently  doubles.  It  will  be  noted  that  the  work  here  sum- 
marized was  all  done  at  approximately  the  same  pressure.  About  as 
many  more  data  were  first  taken  under  various  conditions  of  pressure  to 
determine  the  most  suitable  value  at  which  to  work.  If  the  pressure 
is  a  little  too  low,  too  many  alpha  particles  get  through  and  the  catches 
come  too  frequently.  If  the  pressure  is  too  high,  the  interval  between 
catches  becomes  too  long.  It  was  thus  found  desirable  with  rays  already 
slowed  down  by  aluminum  foil  to  the  extent  these  were,  to  work  with 
air  at  pressures  between  8  and  8.5  cms.  This  corresponded  very  nearly 
to  the  end  of  the  range  of  the  alpha  rays. 

The  column  tg  represents  the  time  taken  for  the  drop  to  fall  50  scale 
divisions,  approximately  5  mm.,  under  gravity  and  thus  indicates, 
roughly,  the  smallness  of  the  drops. 

Tn  another  set  of  observations  of  this  same  kind  in  air  taken  by  other 
observers,  namely  Messrs.  Johnson  and  Rogers,  519  catches  of  positive 
ions  were  made  on  18  drops  under  approximately  the  same  conditions 
as  those  just  summarized.  They  found  15  catches  which  they  counted 
as  doubles  and  5  which,  though  measuring  up  as  doubles,  they  classed  as 
"doubtfuls."  By  a  doubtful  double  they  meant  that  two  jerks  were 
observed  in  the  drop's  motion — a  second  jerk  taking  place  before  the 
shutter  could  be  closed  to  prevent  further  ionization.  While  the  second 
jerk  might  have  been  a  Brownian  movement  jerk,  as  it  often  proved  to 
be  when  measurement  showed  only  a  "single,"  yet  it  was  felt  that  the 
second  jerk  might  have  been  caused  by  the  catching  of  a  second  ion  and 
such  catches  were  therefore  classed  as  doubtful  doubles. 

It  will  be  seen  that  these  two  sets  of  measurements  are  in  very  good 
agreement;  Johnson  and  Rogers  data  showing  about  one  double  in  34 
positive  catches  and  the  data  here  given  showing  one  in  35. 

To  decide  whether  these  were  real  doubles  or  whether  a  large  fraction 
of  these,  too,  were  actually  doubtful  doubles,  it  was  only  necessary  to 
find  what  was  the  average  interval  during  which  the  polonium  shutter 
was  open  before  a  catch  was  made  and  to  calculate  what  the  chance 
would  be  of  two  single  ions  being  caught  in  an  indistinguishably  small 
interval.  The  observations  showed  that  if  the  pressure  was  8.1  cms. 
(in  the  case  of  air),  the  average  exposure  for  a  catch  was  43  sees,  but  when 
the  pressure  rose,  less  than  half  a  cm.,  to  8.5  cms.  of  mercury,  the  interval 
increased  to  167  sees,  and  when  a  pressure  of  9  cm.  was  reached,  no 
catches  were  made  after  several  intervals  of  ten  or  fifteen  minutes,  waiting. 
Hence,  for  alpha  rays  which  had  already  been  slowed  down  by  the  foil 
to  the  extent  which  these  had,  a  pressure  of  between  8.1  and  8.5  cms. 


N^"3  MULTIPLE    VALENCY   IN   ION  I Z  ATI  ON   BY  ALPHA    RAYS.       215 

of  air  enables  us  to  make  measurements  at  practically  the  end  of  the 
range.  The  discussion  of  the  probability  of  two  single  ions  being  caught 
in  an  indistinguishably  small  interval  is  given  below. 

THE  PROBABILITY  FUNCTION. 

It  has  been  shown  repeatedly  that  a  random  distribution  of  ejection 
can  be  assumed  for  the  alpha  rays  emitted  in  radioactive  changes.  But 
it  is  a  familiar  result,  taken  from  the  theory  of  probability,  that  if  a 
series  of  events  happens  with  a  random  distribution  of  intervals,  the 
probability  of  there  being  an  interval  as  long  as  /  seconds  between  two 
successive  events  is 

where  i/u  is  the  average  interval. 

It  was  estimated  that  I  second  was  a  fair  value  to  take  for  the  time 
interval  within  which  it  was  impossible  to  distinguish  with  certainty 
two  successive  catches  as  actually  separate  events  for  it  required  about 
this  time  for  closing  the  shutter  after  a  catch  had  been  seen  to  have 
occurred  and  during  the  act  of  closing  another  catch  might  occur  without 
being  noticeable.  In  the  various  measurements  made,  the  average 
interval  varied  from  13  to  170  sees.  The  probability  function  may  be 
calculated  for  each  interval  but  it  will  be  legitimate  for  the  purpose  in 
hand,  to  consider  only  the  general  average,  namely,  43  sees. 

From  (i),  the  probability  of  an  interval  as  long  as  i  sec.  between  two 
successive  catches  =  e~u.  The  probability  that  there  will  be  an  interval 
between  o  seconds  and  I  second  =  i  —  e~u.  Now 

,  u2      u3   , 

e~u  =  i  -  u  +  —  -  —  +  etc. 
2!      3! 

i  —  e~u  =  u  - 

With  u  =  1/43,  all  terms  of  the  last  expression,  save  the  first,  are  negli- 
gible, i.e.,  the  chance  of  two  successive  catches  following  within  an  interval 
of  i  sec.  is  simply  u. 

It  appears  then  that  since  the  average  interval,  as  given  below,  is 
about  43  sees.,  that  one  catch  in  43  under  such  conditions  should  appear 
as  a  double.  This  number  is  so  near  the  total  number  of  doubles  obtained 
that  we  are  justified  in  the  assertion  that  no  evidence  is  herewith  pre- 
sented for  the  formation  of  doubles  in  air.  We  may  at  least  conclude 
with  certainty  that  the  act  of  ionization  of  air  by  alpha  rays  produces 
no  more  than  two  or  three  per  cent,  of  doubly  charged  ions  and  prac- 
tically never  produces  ions  of  larger  valency  than  2. 


216 


7\   12.    WILKINS. 


: SECOND 
[SERIES. 


HELIUM. 

It  was  decided  to  undertake  quite  an  extensive  investigation  of  helium 
as  no  work  of  this  type  had  been  done  in  that  gas.  Accordingly  some 
2,150  positive  ions  were  caught;  an  attempt  being  made  to  approach 
gradually  to  the  end  of  the  range  of  the  alpha  particles.  This  was  done 
by  gradually  increasing  the  helium  pressure  in  the  oil-drop  chamber  as 
successive  oil  drops  were  blown  in  with  helium.  As  the  "stopping 
power"  of  helium  is  much  less  than  that  of  air,  it  was  found  that  to  slow 
down  the  alpha  particles  to  the  end  of  their  range,  a  much  larger  helium 
pressure  was  required  than  was  necessary  in  air.  The  gas  pressure  was 
steadily  increased  and  measurements  taken  at  various  stages. 

DATA — HELIUM. 

It  will  be  noticed  from  Table  II.  that  the  interval  between  catches 
decreased  at  first  as  the  pressure  was  increased  since  more  atoms  of 
helium  were  present  to  be  ionized.  The  interval  became  a  minimum 


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at  a  pressure  of  about  26.6  cms.  and  then  gradually  increased  indefinitely. 
Thus  the  ionizing  power  of  the  alpha  ray  is  a  maximum  a  short  distance 
from  the  end  of  its  range,  which  is  in  agreement  with  other  work.  (See 
Fig.  i.)  As  the  pressure  was  further  increased,  the  number  of  alpha 
rays  getting  through  was  cut  down  and  so  the  interval  was  lengthened. 


No"!"3XIX]     MULTIPLE    VALENCY   IN  IONIZATION  BY  ALPHA    RAYS.      21 J 


TABLE  II. 


Press. 

No.  of  Posi- 
tives Caught. 

tgof 
Drop. 

Average 
Interval. 

Per  Cent. 
Doubles. 

Average  Per 
Cent.  Doubles. 

Range  Be- 
yond Point 
of  Lip.* 

8.9 

50 

74  sees. 

44  sees. 

2 

2 

4.0  cm. 

13 

50 

73 

23 

4 

4 

3.3 

15 

100 

112 

19 

9 

9 

16 

100 

78 

18 

14 

14 

2.8 

17.8 

100 

98 

17 

12 

12 

2.5 

18.6 
18.7 

50 
50 

99 
109 

18 
13.6 

.      101 

12J 

11 

2.4 

19.4 

50 

113 

20 

12 

12 

2.3 

20.1 

50 

127 

21 

16] 

20.3 

50 

108 

27 

4 

9.3 

2.1 

20.3 

50 

129 

23 

6J 

21.4 

50 

141 

20 

8 

8 

1.9 

22.2 

50 

127 

31 

10 

10 

1.8 

23.5 

50 

124 

28 

12 

12 

1.6 

24.7 

50 

130 

27 

10 

10 

1.4 

25.4 

100 

140 

26 

25.5 

50 

143 

26 

10.5 

1.3 

26.3 

50 

142 

23 

14 

26.4 

50 

161 

29 

18 

14.7 

1.1 

26.6 

100 

155 

17 

12, 

26.8 

150 

163 

26 

9 

26.85 

50 

192 

30 

20  ' 

14 

1.0 

•26.9 

100 

139 

23 

13. 

27.1 

50 

163 

20 

18 

27.3 

50 

157 

39 

2r 

12 

1.0 

27.5 

50 

174 

31 

I 

27.9 

50 

135 

40 

10 

10 

0.8 

28.1 

50 

157 

30 

}- 

28.4 

100 

171 

28 

9 

29 
29.5 

50 
50 

153 
154 

66 
56 

i} 

5 

0.7 

30.95 

50 

138 

67 

2 

2 

0.3 

31.5 

50 

155 

88 

2 

2 

32.5 

50 

148 

154 

2 

2 

0.08 

2150 

The  data  show  distinct  evidence  for  the  formation  of  doubly  charged 
helium  ions  and  show,  moreover,  that  the  formation  of  doubles  does 
indeed  depend  on  the  speed  of  the  ionizing  particle.  The  number  of 
doubles  rises  to  a  maximum  when  the  ionizing  power  of  the  particles  is 
a  maximum  and  amounted,  in  this  work,  to  as  many  as  15  per  cent,  of 

*  Reduced  to  atmospheric  pressure.  This  reduction  was  suggested  and  outlined  by  Dr. 
G.  S.  Fulcher.  It  indicates  that  doubles  are  produced  when  the  ranges  of  the  rays  are  equiv- 
alent to  between  4  and  30  mm.  of  helium  at  atmospheric  pressure. 


218  T.  R.  WILKINS. 

the  number  of  ions  formed.  The  probability  of  two  singles  being  caught 
in  an  indistinguishably  short  interval  would  not  account  for  more  than 
three  or  four  per  cent,  of  the  total  catches  being  doubles,  for  the  average 
interval  at  the  peak  of  the  production  of  doubles  was  about  twenty-five 
seconds  and  thus,  on  the  basis  of  the  reasoning  given  above,  one  ion  in 
twenty-five  might  be  expected  to  be  a  spurious  double.  This  still  leaves 
a  large  margin  for  considering  this  a  real  effect,  and  this  is  made  even 
more  apparent  in  the  light  of  some  remarks  under  the  head  of  hydrogen, 
below. 

Grouping  the  data  corresponding  to  pressures  between  15  and  24  cms., 
it  will  be  seen  that  the  number  of  doubles  is  about  10  per  cent,  of  the 
total  number  of  positive  ions  produced,  but  from  this  region  to  the  end 
of  the  range,  a  marked  rise  and  subsequent  decline  is  noticeable.  Large 
fluctuations  appear  from  this  10  per  cent,  but  this  is  to  be  expected  from 
any  such  probability  measurement  as  this  if  only  a  small  number  of 
observations  is  taken.  There  seemed  no  special  object  in  multiplying 
the  observations  in  this  region,  but  there  was  reason  to  expect  that  the 
region  toward  the  end  of  the  range  might  prove  of  special  interest,  for 
Geiger  had  shown,  as  is  checked  by  the  data  above,  that  toward  the  end 
of  the  range  the  ionization  rises  very  markedly.1  Accordingly,  a  much 
larger  number  of  observations  was  made  in  this  region,  and  Fig.  i  shows 
that  the  curve  is  fixed  quite  definitely  for  this  region  of  interest. 

An  interesting  comparison  may  be  made  between  the  data  on  air  and 
those  on  helium.  It  was  found,  as  already  noted,  that  with  air,  an  interval 
of  167  sees,  ensued  between  catches  if  the  pressure  were  raised  to  8.5 
cms.  Table  II.  shows  that  in  helium  an  interval  of  154  seconds  accom- 
panied a  pressure  of  32.5  cms.  So  the  alpha  particles  were  ionizing 
about  as  infrequently  in  one  case  as  in  the  other.  In  other  words,  8.5 
cms.  of  air  have  about  as  great  stopping  power  for  alpha  particles  as 
32.5  cms.  of  helium.  This  is  in  very  nice  agreement  with  Bragg  and 
Kleeman's  results  that  the  stopping  power  of  an  element  for  alpha  par- 
ticles is  proportional  to  the  square  root  of  the  atomic  weight  and  for 
complex  molecules  is  an  additive  property.2  Adams  has  already  done 
work  on  the  testing  of  this  law  for  helium  and  other  gases.3  Using 
Adams's  calculations  for 

2  VAt.  Weight  air 

=^^^~^~~^^^—~—~^^^~   ==  ^«o 

2  A/At.  Weight  helium 
and  remembering  that  the  stopping  power  is  inversely  proportional  to 

1  Rutherford,  Radioactive  Substances  and  their  Transformations,  p.  153. 

2  Bragg  and  Kleeman,  Phil.  Mag.,  6,  1905,  p.  338. 
'Adams,  PHYS.  REV.,  24,  1907,  p.  108. 


VOL.  XIX. 
No.  3. 


]     MULTIPLE    VALENCY   IN  1ON1ZATION   BY  ALPHA    RAYS.       21 9 


the  pressure,  we  calculate  that  the  pressure  of  helium  necessary  to  stop 
the  alpha  particles  to  the  same  extent  as  would  be  done  by  8.5  cms.  of 
air  would  be  8.5  X  3.8,  or  32.3  cms.  of  helium.  This  is  to  be  compared 
with  the  32.5  cms.  for  approximately  equal  stopping  power  mentioned 

above. 

HYDROGEN. 

The  law  given  by  Bragg  and  Kleeman  for  the  stopping  power  of  gases 
for  alpha  particles  has  been  shown,  above,  to  be  beautifully  checked  in 
the  work  on  helium.  It  follows  from  the  law,  that  hydrogen  should 
have  the  same  stopping  power  as  helium,  for  though  of  different  atomic 
weight,  helium  is  monatomic  and  hydrogen,  diatomic.  If  the  doubles 
in  helium  were  spurious  in  any  way,  the  same  effects  should  occur  in 
hydrogen.  A  series  of  runs  was  therefore  taken  at  that  pressure  at 
which  the  maximum  effect  was  found  in  helium.  Theoretically,  there 
would  be  no  expectation  of  doubles  in  hydrogen  for  the  hydrogen  atom 
is  pictured  as  having  but  one  electron  and  even  though  hydrogen  is 
diatomic,  the  chance  of  catching  a  double,  formed  by  the  electrons  being 
pulled  off  from  each  atom,  would  be  negligible,  for  the  positive  nuclei 
remaining,  would,  presumably,  at  once  repel  each  other  and  not  be  caught 
together.  Commercial  hydrogen  was  used. 

TABLE  III. 


No.  of  Pos. 
Catches. 

Press. 

tg. 

Av.  Interval. 

Doubles. 

Per  Cent. 

50    

26.2 

114 

64  sees. 

1 

2 

100 

26  5 

95 

61 

4 

4 

50  

27 

110 

74 

0 

0 

200 

5 

2.5 

Thus  hydrogen  showed  5  doubles  in  200  but  since  the  probability 
calculation,  as  given  above,  would  class  4  of  these  as  spurious,  it  seems 
fair  to  conclude  that  not  only  were  no  real  doubles  secured  in  hydrogen 
but,  because  of  the  identity  of  stopping  power  and  the  conditions  of 
temperature  and  pressure,  the  evidence  is  confirmed  that  those  secured 
in  helium  were  real  and  in  no  way  due  to  faulty  shielding,  to  the  catching 
of  two  separately  ionized  molecules  or  to  any  such  effects. 

MERCURY  DIMETHYL. 

The  work  of  Millikan,  Gottschalk  and  Kelly  was  repeated  in  this  case 
also  but  no  evidence  for  real  doubles  was  secured.  The  delicate  adjust- 
ment for  the  end  of  the  range  could  not  be  made,  in  the  case  of  this 


220  T.   R.    WILKINS. 

poisonous  vapor,  as  with  the  other  gases,  so  the  vapor  pressure  was 
raised  by  raising  the  room  temperature.  The  multiple  relationship 
between  the  different  valence  speeds  shown  in  the  first  part  of  the  paper 
proved  of  great  value  here  as  the  characteristic  speeds  constantly  changed 
with  the  vapor  pressure  changes. 

In  conclusion,  the  writer  wishes  to  thank  Professor  R.  A.  Millikan  for 
outlining  the  problem  and  for  his  valuable  suggestions  during  the  course 
of  the  work.  The  present  paper  is  but  a  continuation  of  Professor 
Millikan's  earlier  papers  as  has  been  indicated  in  the  references  above. 

RYERSON  LABORATORY, 

UNIVERSITY  OF  CHICAGO. 


520420 


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